1. Field of the Invention
From a technological standpoint, a major problem associated with the nitric acid acidulation of phosphate rock, as currently practiced, involves the production of the soluble byproduct, calcium nitrate, which renders the fertilizer products hygroscopic and generally difficult to store. Furthermore, the presence of soluble calcium causes the reversion of phosphate to low-water and citrate-soluble forms (dicalcium phosphate and apatite) when the resulting acidulates are subsequently neutralized with ammonia and granulated to produce solid N-P fertilizer products. In practice, these problems may be circumvented by removal of part of the calcium by refrigeration-crystallization (Odda Process); by reaction of the calcium with supplemental phosphoric acid (Phosphonitric Process); or by precipitation with sulfate (Sulfonitric Process) [A. V. Slack et al., Farm Chemicals, Vol. 130, Nos. 4, 5, 6 and 7 (April, May, June, and July, 1976)]. However, refrigeration-crystallization processes are complex and require high capital investment per ton of product and high operating cost because of high energy requirements. Also, it is difficult to economically dispose of some of the byproducts (calcium carbonate or gypsum) from the other processes.
2. Prior Art
A number of prior-art investigations concerning the reactions of nitric acid and urea with phosphate ore have been reported in the scientific and patent literature. Initial work was conducted by Nagai and his coworkers in the early 1950s [S. Nagai and T. Kanasawa, J. Chem. Soc., Japan, Ind. Chem. Sect., 54, 39 (1951); S. Nagai, T. Kanasawa, and K. Saito, J. Chem. Soc. Japan Ind. Chem. Sect., 54, 210 (1951); S. Nagai, T. Kanasawa, and K. Sakurada, J. Chem. Soc. Japan, Ind. Chem. Sect., 54, 611 (1951)]. These workers investigated the properties of reaction products produced in the pure component system Ca.sub.3 (PO.sub.4).sub.2 --CO(NH.sub.2).sub.2 --HNO.sub.3 and determined that the urea adducts: "cal urea" [CaNO.sub.3.4CO(NH.sub.2).sub.2 ], and urea nitrate [CO(NH.sub.2).sub.2.HNO.sub.3 ] were the primary reaction products. They further extended their studies to include the addition of various quantities of urea to nitric acid acidulates of phosphate ore and concluded that products containing about 24- 25 percent total N (urea-N.about.19-20 percent, NO.sub.3 -N.about.4.5 percent) and about 9-10 percent P.sub.2 O.sub.5 offered the best physical properties for consideration as possible N-P fertilizer products. However, the critical relative humidities of these materials were not reported.
Japanese patents No. Sho 26-767, February 1951, and No. Sho 28-3169, July 1951, describe processes for the acidulation of phosphate ore with nitric acid followed by the addition of urea and calcium or potassium salts to produce nonhygroscopic granular fertilizers.
Guillet et al, in French Pat. No. 1,107,730, January 1956, describe the reaction of urea with hydrated calcium nitrate containing nitric and phosphoric acids to produce Ca(NO.sub.3).sub.2.4CO(NH.sub.2).sub.2. This product was used in combination with Ca.sup.+2, NH.sub.4.sup.+1, and P.sub.2 O.sub.5 containing salts to produce N-P fertilizers. Further work (French Pat. No. 1,182,058, June 1959) describes the reaction of phosphate rock with HNO.sub.3 and urea to give an improved N-P fertilizer containing a complex mixture of urea calcium nitrate [Ca(NO.sub.3).sub.2.(CO(NH.sub.2).sub.2).sub.4 ], urea phosphate [CO(NH.sub.2).sub.2.H.sub.3 PO.sub.4 ], urea nitrate [CO(NH.sub.2).sub.2.HNO.sub.3 ], and dicalcium phosphate (CaHPO.sub.4). The improved physical properties of this N-P fertilizer material resulted from the formation of urea adducts with nitrate, phosphate, and calcium present within the solid reaction mixture.
In 1963 Russian investigators [Ya. S., Shenken, V. A. Klevke, and B. G. Lyudkovskaya, Dokl. Akad. Nuak. SSSR 149, 656-9 (1963)] studied mixtures of calcium nitrate, phosphoric acid, and urea at 1:1 ratios of urea to phosphoric acid and calcium nitrate. The primary products were urea nitrate [CO(NH.sub.2).sub.2.HNO.sub.3 ] and monocalcium phosphate [Ca(H.sub.2 PO.sub.4).sub.2.H.sub.2 O]. Addition of more urea resulted in the formation of Ca(NO.sub.3).sub.2.4CO(NH.sub.2).sub.2.
In 1964 this Russian study, supra, was extended to include an x-ray and thermodynamic investigation [Ya. S. Shenkin and V. A. Klevke, Khim. Prom-st, No. 1, 57 (1964)] of various Ca(NO.sub.3).sub.2.4H.sub.2 O, H.sub.3 PO.sub.4, and CO(NH.sub.2).sub.2 mixtures. This study again showed presence of CO(NH.sub.2).sub.2.HNO.sub.3, Ca(NO.sub.3).sub.2.4CO(NH.sub.2).sub.2, CO(NH.sub.2).sub.2.H.sub.3 PO.sub.4, CaHPO.sub.4, and Ca(H.sub.2 PO.sub.4).sub.2.H.sub.2 O as products of the reactions and also established the existence of retrograde equilibria such as: Ca(H.sub.2 PO.sub.4).sub.2 +Ca(NO.sub.3).sub.2 +2CO(NH.sub.2).sub.2 .revreaction.2CaHPO+2[CO(NH.sub.2).sub.2.HNO.sub.3 ]; Ca(H.sub.2 PO.sub.4).sub.2 +2[CO(NH.sub.2).sub.2.HNO.sub.3 ]+4CO(NH.sub.2).revreaction.(Ca(NO.sub.3).sub.2.4CO(NH.sub.2).sub.2 +2[CO(NH.sub.2).sub.2.H.sub.3 PO.sub.4 ]; and CaHPO.sub.4 +2[CO(NH.sub. 2).sub.2.HNO.sub.3 ]+3CO(NH.sub.2).sub.2 .revreaction.Ca(NO.sub.3).sub.2.4CO(NH.sub.2).sub.2 +CO(NH.sub.2).sub.2.H.sub.3 PO.sub.4 as water is removed from the system to produce solid products. However, these investigators failed to identify the presence of the new triple component adduct, Ca(H.sub.2 PO.sub.4)(NO.sub.3).CO(NH.sub.2).sub.2 discovered during the present investigation. This new compound has a very high critical relative humidity (69 percent), almost as good as pure urea (75 percent), which improves the physical properties of the final products tremendously. In addition to this, P.sub.2 O.sub.5 in the compound is 92 percent water soluble and completely available to plant growth (citrate solubility=100 percent). Also, the new compound is acidic when dissolved in water (pH=3.08) such that it possesses all the benefits of other acidic fertilizers.
Subsequently, Frazier et al (A. W. Frazier, J. R. Lehr, and J. P. Smith, Agr. and Food Chem., Vol. 15, No. 2, page 345, Mar./Apr. 1967) were able to isolate and characterize a new double salt adduct, urea-monocalcium phosphate [Ca(H.sub.2 PO.sub.4).sub.2.4CO(NH.sub.2).sub.2 ], from CaO-P.sub.2 O.sub.5 -CO(NH.sub.2).sub.2 -H.sub.2 O systems.
Kochemba et al, U.S.S.R. 775,096, October 1980, concerned themselves with the reaction of phosphate rock with nitric acid followed by the addition of a SO.sub.4.sup.- component and urea to produce a N-P fertilizer product. This work was followed by a publication (Yu. I., Kochemba and Ya. S. Shenkin, Zhurnal Prikladonoi Khimii, Vol. 55, No. 5, pp. 977-980, May 1982) in which KCl and NH.sub.3 are incorporated into the reaction products to produce a urea-N-P-K fertilizer.
Stoller, U.S. Pat. No. 4,315,763, describes some of the applications and agronomic advantages of low pH solution fertilizers (&lt;4.0). The Tennessee Valley Authority has conducted extensive research using low pH urea phosphate fertilizers for application on different types of soils and this research generally shows that there is an agronomic advantage for the use of low pH fertilizer on some soil systems. The products from the instant invention not only have a low pH, but they also contain the highly valuable nitrate nitrogen source. Nitrate nitrogen is a form of nitrogen which is less likely to be lost by evaporation and more likely to be absorbed directly by the roots of a growing crop.